If the Giant’s power and/or toughness change so that its toughness is less than its power, you can have the ability target the Giant itself.

10/1/2009

To work as an evasion ability, an attacking creature must already have flying when the declare blockers step begins. Once a creature has become blocked, giving it flying won’t change that.

10/1/2009

If Stone Giant’s ability is activated during a turn’s end step, the delayed triggered ability won’t trigger until the beginning of the next turn’s end step. The targeted creature will be destroyed at that time.

10/1/2009

When the delayed triggered ability resolves, the targeted creature is destroyed, even if it’s no longer a creature, no longer under your control, or no longer has toughness less than Stone Giant’s power at that time.